Nitrate loads in sub-tropical headwater streams driven by intensive horticulture

Abstract

Nitrogen runoff from fertiliser intensive land uses has become an issue worldwide, contributing to algal blooms, hypoxic waters and aquatic biodiversity losses. This study assessed potential nutrient pollution from blueberry farms in subtropical Australia and examines whether nutrient loads were driven by groundwater discharge and/or surface water runoff. Streams downstream of eight blueberry farms were compared to eight nearby control sites without any blueberry activity. In the 90 day sample period, there were three rain events >90 mm day−1 that produced runoff sufficient to create flooding. Overall, the results revealed a clear link between blueberry farming and nitrogen runoff in headwater streams. While NOX (nitrate + nitrite) was the dominant nitrogen species downstream of blueberry farms, dissolved organic nitrogen (DON) was the dominant species in control sites. The concentrations and loads of NOx were one order of magnitude lower in the eight non-blueberry (6.3 ± 2.0 μmol L−1; 1.6 ± 1.2 kg N-NOX ha−1 yr−1) than the eight blueberry (56.9 ± 14.2 μmol L−1; 21.8 ± 8.0 kg N-NOX ha−1 yr−1) sites. NOX concentrations and loads were highest following rain events. Radon (222Rn, a natural groundwater tracer) observations and low nitrogen concentration in groundwater samples further suggest that surface runoff dominates the delivery of nitrogen to the creeks investigated. NOX concentrations and loads in creeks correlated with blueberry farm density. At >15% of blueberry land use in a catchment, there was a detectable influence in NOX concentrations and loads in the headwater streams. Assuming that our load estimates can be up-scaled to annual nitrogen creek exports, and that local farmers use the recommended amount of fertiliser (121 kg N ha−1 yr−1), between 18 and 25% of the used fertiliser was lost to the creeks. This implies that there are opportunities for decreasing the use of fertilisers in this catchment and managing any nitrogen that escapes to the creeks.